1. Field of the Invention
The present invention relates to an optical fiber composite in which two types of optical fibers having a chromatic dispersion opposite in sign to each other at the wavelength of a signal-carrying lightwave are spliced in cascade, an optical fiber cable comprising a plurality of the optical fiber composites, and methods for producing the optical fiber composite and the optical fiber cable.
2. Description of the Background Art
Optical wavelength division multiplexing (WDM) transmission systems enable large-capacity optical communication by using a plurality of wavelengths. Optical fiber transmission lines to be installed in the repeater sections of a WDM transmission system are required to have an excellent transmission property in the wavelength band of the signal-carrying lightwave, for example, a 1.55 xcexcm-wavelength band. In order to meet this requirement, researchers and engineers have proposed optical fiber transmission lines having an ununiform transmission property along the length of the transmission line.
For example, a first literature (T. Naito, et al., xe2x80x9c1 Terabit/s WDM Transmission over 10,000 km,xe2x80x9d ECOC""99, PD2-1, 1999) states an optical fiber transmission line consisting of the first optical fiber placed at the upstream side and the second optical fiber placed at the downstream side in terms of the propagation direction of a signal. The first optical fiber has a positive chromatic dispersion and a relatively large mode-field diameter at a 1.55-xcexcm-wavelength band. The second optical fiber, a dispersion-compensating optical fiber (DCF), has a negative chromatic dispersion and a relatively small mode-field diameter at a 1.55-xcexcm-wavelength band.
With this optical fiber transmission line, although a signal-carrying lightwave is intense when it starts to propagate in the first optical fiber, nonlinear optical phenomena seldom occur because the first optical fiber has a relatively large mode-field diameter. The lightwave decreases its intensity during the propagation through the first optical fiber. Because the lightwave with decreased intensity propagates through the second optical fiber, nonlinear optical phenomena seldom occur despite the second optical fiber having a relatively small mode-field diameter. In addition, because the two optical fibers have a chromatic dispersion opposite in sign to each other, a proper selection of the ratio of the lengths between the two optical fibers enables the overall chromatic dispersion of the entire optical fiber transmission line to be nearly zero. As described above, signal degrading due to the generation of nonlinear optical phenomena and a large value of overall chromatic dispersion can be avoided by using an optical fiber transmission line in which the first optical fiber and the second optical fiber are spliced in cascade in terms of the propagation direction of a signal-carrying lightwave.
A second literature, U.S. Pat. No. 5,894,537 entitled xe2x80x9cDispersion Managed Optical Waveguide,xe2x80x9d discloses an optical fiber transmission line in which a section having a positive chromatic dispersion at the wavelength band of a signal-carrying lightwave and another section having a negative chromatic dispersion at the same wavelength band are provided alternately and successively in the longitudinal direction. Because both sections are designed to have a large absolute value in chromatic dispersion, the occurrence of nonlinear optical phenomena such as four-wave mixing can be suppressed. Furthermore, because the overall chromatic dispersion of the entire optical fiber transmission line is designed to be small in absolute value, the degradation in the transmission quality can be suppressed.
An object of the present invention is to offer an optical fiber composite easily provided with a desired transmission property as a whole even after a length of optical fiber is cut off from one or both ends, an optical fiber cable comprising the optical fiber composites, and methods for producing the optical fiber composite and the optical fiber cable.
In order to achieve this object, the present invention offers an optical fiber composite in which the following three optical fibers are spliced in cascade in this order:
(a) the first optical fiber having a first chromatic dispersion, D1, at the wavelength of a signal-carrying lightwave;
(b) the second optical fiber having a second chromatic dispersion, D2, at the wavelength of the signal-carrying lightwave, with the second chromatic dispersion having the sign opposite to that of D1; and
(c) the third optical fiber that has the same chromatic dispersion, D1, as the first optical fiber at the wavelength of the signal-carrying lightwave and that is shorter than the first optical fiber.
The optical fiber composite is produced by the following steps:
(a) determining the length, L2, of the second optical fiber by using the following parameters:
the chromatic dispersion, D1, of the first and third optical fibers;
the chromatic dispersion, D2, of the second optical fiber;
the designed length, Lt, of the optical fiber composite; and
the designed mean chromatic dispersion, Dm, of the optical fiber composite;
(b) splicing the second optical fiber, having the length L2, to one end of the first optical fiber and splicing the third optical fiber to the free end of the second optical fiber; and
(c) cutting off a length of optical fiber from at least one free end of the first and third optical fibers so that the total length of the optical fiber composite can be adjusted to the designed length, Lt, with the third optical fiber being shorter than the first optical fiber.
In an embodiment of the above-described optical fiber composite; the ratio of the length, L3, of the third optical fiber to the length, L1, of the first optical fiber (L3/L1) may be 0.1 or less; or the length, L3, of the third optical fiber may be 1 km or less. Furthermore, the absolute value of the chromatic dispersion D2 may be larger than that of chromatic dispersion D1.
The optical fiber cable of the present invention is a bundle of a plurality of the above-described optical fiber composites and is produced by the following steps:
(a) determining the length, L2, of the second optical fiber by using the following parameters:
the chromatic dispersion, D1, of the first and third optical fibers;
the chromatic dispersion, D2, of the second optical fiber;
the designed length, Lt, of the optical fiber composite; and
the designed mean chromatic dispersion, Dm, of the optical fiber composite;
(b) producing an optical fiber composite by splicing the second optical fiber, having the length L2, to one end of the first optical fiber and splicing the third optical fiber to the free end of the second optical fiber;
(c) forming an optical fiber cable by bundling together a plurality of the optical fiber composites; and
(d) cutting off a length of optical fiber from at least one free end of the first and third optical fibers of each of the optical fiber composites in the optical fiber cable so that the total length of the optical fiber composite can be adjusted to the designed length, Lt, with the third optical fiber being shorter than the first optical fiber.
The present invention is further explained below by referring to the accompanying drawings. The drawings are provided solely for the purpose of illustration and are not intended to limit the scope of the present invention.